Further studies concerning the detoxification of noxious, lipophilic foreign and endogenous compounds through oxidation by monooxygenase activity and/or through conjugation by UDP glucuronosyltransferase activity have continued primarily with the ubiquitous environmental pollutant, benzo(a)pyrene, and bilirubin. With biochemicaaly produced 1-hydroxy-benzo (a)pyrene as substrate for further metabolism in vitro studies with hepatic microsomes indicate that: (1) certain monooxygenase activity further metabolizes the phenol to highly mutagenic products as well as generates a DNA binding profile very similar to that for the carcinogenic intermediates of benzo(a)pyrene and that (2) glucuronidation of the phenol by transferase protects against mutagenesis and partly against DNA binding. With bilirubin as substrate, characterization studies with immunoprecipitates of transferase by specific aatibodies indicate that induced blirubin transferase activity in hydrocarbon responsive C57BL/6N mice at 10 days after 3-methylcholanthrene treatment is highly labelled in vivo by pulses of radiolabelled amino acids, whereas transferase in the hydrocarbon nonresponsive DBA/2N mice is not significantly labelled under the same conditions. Further, characterization of other induced bilirubin transferases are under investigation.